Different indicators of acidic
With universal indicators, the concentration of protons or H3O+ ions can be indicated by a defined colour. Thus, with a usual universal indicator - domyhomework , a high proton concentration (acidic) leads to a red colouring and a low proton concentration (basic) to a blue colouring. In the neutral range, it is coloured yellowish/green.
However, there are different indicators that show different colour reactions in the acidic or basic range. Their "transition points", at which they change colour, do not necessarily have to be at a pH value of 7.
pH value of strong acids and bases
With strong acids and bases, the calculation of the pH value is very simple. Strong acids and bases are practically completely dissociated. Thus, the concentration of H3O+ ions or OH- ions is equal to the concentration of the acid used.
For an aqueous solution of a strong acid, e.g. hydrochloric acid, of the concentration of 0.01 mol/l, the following thus applies:
HA + H2O → H3O^+ + A^- c(H3O^+) = c(HA) = 0.01 mol/l = 10^-2 mol/lpH = -lg c(H3O^+) = 2
pH value of weak acids and bases
Weak acids and bases do not dissociate completely. Therefore, one cannot assume that the concentration of protons or H3O+ ions corresponds to the initial concentration of the acid, because only a part of the acid molecules (HA) are dissociated - urgent essay writing service . The pKs value is defined as a measure of the strength of the dissociation (acid strength). Analogous to the pH value, the pKs value is the negative decadic logarithm of the acid constant KS. The constant KS is the equilibrium constant for the dissociation of the acid. The following relationship thus results from the MWG:
HA + H2O ⇌ H3O^+ + A^-
KS' = (c(H3O^+)⋅c(A^-))/(c(HA)⋅c(H2O)) since c(A^-) = c(H3O^+),
KS'[c(H2O)] = (c^2(H3O^+))/(c(HA)) = KS
KS[c(HA)] = c^2(H3O^+)
(pKS-lgc(HA))/2 = pH
In this equation only the concentration of the acid in equilibrium c(HA) is unknown. It results from the difference between the initial concentration of acid and the dissociated part c(A-) - chemistry problem solver . However, since dilute weak acids dissociate so little, i.e. c(A-) << c(HA), one can assume that the initial concentration of the acid is almost equal to the concentration of the undissociated acid in equilibrium.